Method with trench source to increase the coupling of source to floating gate in split gate flash

ABSTRACT

A split-gate flash memory cell having improved programming and erasing speed with a tilted trench source, and also a method of forming the same are provided. This is accomplished by forming two floating gates and their respective control gates sharing a common source region. A trench is formed in the source region and the walls are sloped to have a tilt. A source implant is performed at a tilt angle and the trench is lined with a gate oxide layer. Subsequently, a lateral diffusion of the source implant is performed followed by thermal cycling. The lateral enlargement of the diffused source is found to increase the coupling ratio of the split-gate flash memory cell substantially.

BACKGROUND OF THE INVENTION

[0001] (1) Field of the Invention

[0002] The present invention relates to the manufacture of semiconductormemories, and in particular, directed to a split-gate flash memoryhaving an increased coupling ratio of source to floating gate through ajudicious tilt angle implanting in a trench source with tilted walls,and to a method of forming of the same.

[0003] (2) Description of the Related Art

[0004] Normally, a high degree of coupling is desired between the sourceand the floating gate of a split-gate flash memory cell in order toprovide enhanced erasing and programming speed, as is known in the art.If the high degree of coupling is sought by higher implant energy, thenthe floating gate gets damaged. If, on the other hand, the increase inthe coupling ratio is attempted by increasing the lateral diffusion ofthe implant, then the well-known problems of punch-through and junctionbreakdown are encountered. These problems are not unique to flat orshallow source regions only. Even with a three dimensional trench butstraight walled source regions, same problems are encountered unlessadditional steps are taken, as disclosed later in the embodiments of thepresent invention.

[0005] Over the years, numerous improvements in the performance as wellas in the size of memory devices have been made by varying the simple,basic one-transistor memory cell, which contains one transistor and onecapacitor. The variations consist of different methods of formingcapacitors, with single, double or triple layers of polysilicon, anddifferent materials for the word and bit lines. In general, memorydevices include electrically erasable and electrically programmableread-only memories (EEPROMs) of flash electrically erasable andelectrically programmable read-only memories (flash EEPROMs). Many typesof memory cells for EEPROMs or flash EEPROMs may have source and drainsregions that are aligned to a floating gate or aligned to spacers. Whenthe source and drain regions are aligned to the floating gate, a gateelectrode for a select transistor is separate from the control gateelectrode of the floating gate transistor. Separate select and controlgates increase the size of the memory cell. If the source and drainregions are aligned to a spacer formed after the floating gate isformed, the floating gate typically does not overlie portions of thesource and drain regions. Programming and erasing performance isdegraded by the offset between the floating gate and source and drainregions.

[0006] Most conventional flash-EEPROM cells use a double-polysilicon(poly) structure of which the well known split-gate cell is shown inFIG. 1. Here, two MOS transistors share a source (25). Each transistoris formed on a semiconductor substrate (10) having a first doped region(20), a second doped region (25), a channel region (23), a gate oxide(30), a floating gate (40), intergate dielectric layer (50) and controlgate (60). Substrate (10) and channel region (23) have a firstconductivity type, and the first (20) and second (25) doped regions havea second conductivity type that is opposite the first conductivity type.

[0007] As seen in FIG. 1, the first doped region, (20), lies within thesubstrate. The second doped region, (25), also lies within substrate(10) and is spaced apart form the first doped region (20). Channelregion (23) lies within substrate (10) and between first (20) and second(25) doped regions. Gate oxide layer (30) overlies substrate (10).Floating gate (40), to which there is no direct electrical connection,and which overlies substrate (10), is separated from substrate (10) by athin layer of gate oxide (30) while control gate (60), to which there isdirect electrical connection, is generally positioned over the floatinggate with intergate oxide (50) therebetween.

[0008] In prior art, different methods for increasing the couplingbetween the source and the floating gate are taught. In U.S. Pat. No.6,159,801, Hsieh, et al., disclose a three-dimensional source capable ofthree-dimensional coupling with the floating gate of a split-gate flashmemory cell. This is accomplished by first forming an isolation trench,lining it with a conformal oxide, then filling with an isolation oxideand then etching the latter to form a three-dimensional coupling regionin the upper portion of the trench. A floating gate is next formed byfirst filling the three-dimensional region of the trench withpolysilicon and etching it. The control gate is formed over the floatinggate with an intervening inter-poly oxide. The floating gate forms legsextending into the three-dimensional coupling region of the trenchthereby providing a three-dimensional coupling with the source whichalso assumes a three-dimensional region. The leg or the side-wall of thefloating gate forming the third dimension provides the extra areathrough which coupling between the source and the floating gate isincreased. In U.S. Pat. Nos. 6,017,795 and 6,124,609, Hsieh, et al.,propose a different split-gate flash memory cell with increased couplingratio, and the making of the same. Here, the source line is formed in atrench in a substrate over a source region. The trench walls provide theincrease source in the coupling.

[0009] Kim of U.S. Pat. No. 5,527,727, on the other hand, discloses amethod of manufacturing a split-gate EEPROM cell where an active regionis defined to include a source bit line and a drain bit line region. Afirst polysilicon layer is etched through a floating gate mask until asilicon substrate in the source bit line region and the drain bit lineregion is exposed. A buried N+ layer is formed in the exposed siliconsubstrate by implanting impurity ions. A thick oxide film is formed onthe buried N+ layer by a subsequent oxidation process, and this thickoxide film is etched to a constant thickness by a self-aligned etchingprocess for forming a float gate. Thereafter, a select gate oxide filmand a select gate are formed by a general process. Thus, the electricalcharacteristics of the cell is enhanced by decreasing the topologygenerated by the oxide film formed in a bit line containing a sourceregion and a drain region, and a bit line is formed containing a sourceregion and a drain region by performing the buried N+ impurity ionimplantation process only once.

[0010] In addition, fabrication of a non-volatile memory is described byLee, et al., in U.S. Pat. No. 6,037,221 while Ogura describes the makingof a non-volatile random access memory in U.S. Pat. No. 5,780,341.

[0011] The present invention discloses still a different method offorming a split-gate flash memory device characterized by a split-gateside (between the control gate and the drain), a stacked-side (betweenthe floating gate and the source) and by a coupling ratio between thefloating gate and the source. As is stated earlier, the coupling ratioaffects the program speed, that is, the larger the coupling ratio, thefaster is the programming speed, and is not a fixed value by virtue ofthe variability of the channel length and hence that of the overlapbetween the floating gate and the source. Usually, if channel length isincreased through greater lateral diffusion in the source region,punchthrough occurs due to excessive current well below the thresholdvoltage. It is shown later in the embodiments of the present inventionthat the coupling ratio can be increased by incorporating a judicioustilt angle implant in a trench source having tilted walls, thusalleviating the punchthrough and junction break-down of the sourceregion.

SUMMARY OF THE INVENTION

[0012] It is therefore an object of the present invention to provide amethod of forming a split-gate flash memory with a trench source havingan increased coupling to the floating gate.

[0013] It is still another object of this invention to provide a methodof forming a trench having tilted walls for increased coupling of thesource to the floating gate of a split-gate flash memory cell.

[0014] It is yet another object of the present invention to provide asplit-gate flash memory cell with a trench source having an increasedcoupling to the floating gate.

[0015] It is an overall-object of this invention to provide a split-gateflash memory cell having improved programming and erasing speed with atrench source, and also a method of forming the same.

[0016] These objects are accomplished by providing a substrate havingactive and passive regions defined; forming a first gate oxide layerover said substrate; forming a first polysilicon layer over said gateoxide layer; forming a nitride layer over said first polysilicon layer;patterning said nitride layer to expose a portion of said firstpolysilicon layer to define a floating gate area; performing oxidationof said portion of said first polysilicon layer to form a polyoxidelayer over said first polysilicon layer; etching said first polysiliconlayer using said polyoxide layer as a hard mask to form a floating gate;forming an interpoly oxide over said polyoxide; forming a secondpolysilicon layer over said interpoly oxide; patterning said secondpolysilicon layer to form a control gate; forming a trench source insaid substrate; performing a source implant; forming a second gate oxidelayer over the inside walls of said trench source; performing a lateraldiffusion of said source implant; and performing thermal cycle of saidsubstrate.

[0017] These objects are further accomplished by providing a substratehaving a source region; a split-gate flash memory cell on saidsubstrate; a trench source in said source region; a gate oxide layerover the inside walls of said trench source; and a laterally enlargeddiffused area of said source region.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a cross-sectional view of a conventional split-gate typememory cell of prior art.

[0019]FIGS. 2a-2 i are top views of a substrate showing the forming of asplit-gate flash memory cell of this invention having a trench sourcewith improved coupling to the floating gate.

[0020]FIGS. 3a-3 i correspond to the top views of FIGS. 2a-2 i showingthe cross-sections of the substrate of the present invention,specifically:

[0021]FIG. 3a is a cross-sectional view of the substrate of FIG. 2ashowing the forming of first gate oxide layer, according to the presentinvention.

[0022]FIG. 3b is a cross-sectional view of the substrate of FIG. 2bshowing the forming of a first polysilicon layer, followed by theforming of a nitride layer, and a first photoresist layer, according tothe present invention.

[0023]FIG. 3c is a cross-sectional view of the substrate of FIG. 2cshowing the patterning of the first photoresist layer, according to thepresent invention.

[0024]FIG. 3d is a cross-sectional view of the substrate of FIG. 2dshowing the patterning of the nitride layer, according to thisinvention.

[0025]FIG. 3e is a cross-sectional view of the substrate of FIG. 2eshowing the forming of the polyoxide caps of the floating gate as wellas the floating gate itself, according to the present invention.

[0026]FIG. 3f is a cross-sectional view of the substrate of FIG. 2fshowing the forming of an interpoly oxide layer, comprising a layer ofhigh temperature oxide sandwiched between two layers of thermal oxide,according to the present invention.

[0027]FIG. 3g is a cross-sectional view of the substrate of FIG. 2gshowing, after the forming of the control gates, the forming of thetrench source of the present invention, with tilted walls.

[0028]FIG. 3h is a different cross-sectional view of the substrate ofFIG. 2h showing the increased lateral diffusion of the trench source ofthis invention after annealing of the substrate.

[0029]FIG. 3i is an additional cross-sectional view of the substrate ofFIG. 2i showing the further increase in the lateral diffusion of thetrench source of this invention after thermal cycling of the substrate,thereby increasing the coupling ratio between the source and thefloating gate, according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030] Referring now to the drawings, specifically to Figs. FIGS. 2a-2i, and FIGS. 3a-3 i, there is shown a method of forming a split-gateflash memory cell having improved programming and erasing speed with atilted trench source, and also a structure thereof. FIGS. 2a-2 i show atop view of a semiconductor substrate at different steps of the process,while FIGS. 3a-3 i show the cross-sectional views of the substrate atthe corresponding steps.

[0031] Thus, FIG. 2a shows a top view of a semiconductor substrate (100)where active regions (107) and field regions (105) have been defined. Across-section through an active region is shown in FIG. 3a. First, alayer of gate oxide (110), better seen in the cross-sectional view, isformed over the substrate. This first gate oxide layer may be formed byusing chemical vapor deposition (CVD) SiO₂, or grown thermally. It ispreferred that layer (110) is grown thermally at a temperature betweenabout 800 to 950° C., and to a thickness between about 70 to 90angstroms (Å).

[0032] Next, first polysilicon layer (120), later to be formed into afloating gate, is deposited over the first gate oxide layer, as shown inFIGS. 2b and 3 b. Polysilicon is formed through methods including butnot limited to Low Pressure Chemical Vapor Deposition (LPCVD) methods,CVD methods and Physical Vapor Deposition (PVD) sputtering methodsemploying suitable silicon source materials. It is preferred that LPCVDis used with a silicon source SiH₄ at a temperature between about 530 to650° C. This is followed by forming nitride layer (130) over the firstpolysilicon layer. It is preferred that nitride layer is formed by CVDat a temperature between about 650 to 800° C. by reacting dichlorosilane(SiCl₂H₂) with ammonia (NH₃) and to a thickness between about 700 to 900Å. Then, first photoresist layer (140) is formed and patterned as shownin FIGS. 2b and 3 b. Openings (145), where floating gates are defined,can be better seen in the cross-sectional view in FIG. 3c.

[0033] Nitride layer (130) is then etched. The etch stops on thepolysilicon layer, as shown in FIG. 3d. In the top view in FIG. 2d,portions of the silicon layer that are exposed at the bottom of theetched openings are shown. At the next step photoresist layer (140) isremoved.

[0034] Through the patterned openings in the nitride layer, exposedpolysilicon is next oxidized using wet-oxidation at a temperaturebetween about 800 to 950° C. The resulting polyoxide layer, or “caps”(125), are shown in FIG. 3e, where the nitride layer is no longer neededand has been wet-stripped in phosphoric acid solution H₃PO₄. Thepolyoxide layer preferably has a thickness between about 1100 to 1300 Å.Using the polyoxide layer as a hard mask, the polysilicon layer isetched, thus forming floating gates (120) which are shown in FIG. 3e,and the overlying “caps” (125) in the top view in FIG. 2e.

[0035] A composite interpoly oxide layer (150) is next formed over thefloating gate as shown in FIG. 3f. The top view is shown. in FIG. 2f.The composite layer comprises three layers where the first layer is afirst tnermal oxide which is thermally grown at a temperature betweenbout 800 to 950° C., and to a thickness between about 30 to 50 Å. Thesecond layer is a high temperature oxide (HTO), deposited to a thicknessbetween about 120 to 140 Å at a temperature between about 800 to 950° C.And the third layer is a second thermal oxide layer, also grown at thesame temperature as the first gate oxide layer, but to a thicknessbetween about between about 60 to 80 Å. The preferred total thickness ofinterpoly oxide layer (150) in FIG. 3f is thus between about 210 to 270Å.

[0036] Subsequently, using the same process as in forming the firstpolysilicon layer, a second polysilicon layer (160) is formed over theinterpoly oxide layer. Then, following the normal process steps offorming and patterning another photoresist layer (not shown) to definethe control gate, and etching the second polysilicon layer to form thecontrol gate, a structure is formed as shown in the cross-sectional viewin FIG. 3g. The preferred thickness of the second polysilicon layer isbetween about 1900 to 2100 Å.

[0037] After the removal of the photoresist layer to form the controlgates, another second photoresist, layer (170) in FIGS. 2g and 3 g, isformed over the control gate to define cell source area. Then, and as amain feature and key aspect of the present invention, the source regionis etched to form a trench source. Trench source (109) is also shown inFIG. 3g and has a depth between about 220 to 600 Å. It is important,however, that the trench also has tilted walls with an included angle αbetween about 10 to 45 degrees. Taking advantage of tilted walls, asource implant is performed at its tilt angle between about 10 to 45degrees with phosphorous ions at a dosage level between about 1×10¹⁵ to1×10¹⁶ atoms/cm², and energy between about 10 KeV to 50 KeV.Subsequently, a second thermal oxide, layer (190) in FIG. 3h, is formedover the tilted walls of the trench source. This is accomplished bythermal growth at a temperature between about 800 to 950° C., and to athickness between about 60 to 80 Å.

[0038] It will be noted, however, that although the coupling range (187)of the diffusion area (185) of the trench source, as obtained with thedisclosed tilt angles, is wider than the conventional ranges obtainedwith flat source and implant, it is disclosed here that the range can beimproved even further by a subsequent critical step. This involves afurther lateral diffusion of implanted ions by annealing the substrateat a temperature between about 800 to 950° C. It is found that thelateral diffusion can be improved even more by subjecting the substrateto thermal cycling as depicted by the reference numeral (200) in FIG.3h. That is, the range of the newly diffused area (205) spans at leastone-half the width of the floating gate, namely, reaching point (207) asshown in both FIGS. 3h and 3 i. The cross-sectional views are that ofthe top views given in FIGS. 2h and 2 i. The thermal cycle isaccomplished between temperatures about 800 and 950.

[0039] Thus, the disclosed tilted trench source provides a highercoupling ratio of source to floating gate with lower implant energy thanis possible with conventional flat source cells. This is primarilybecause of the increased lateral diffusion area of the straggle or strayions assisted by the tilt angle of both the sidewalls of the trench aswell as the tilt angle implant of the source, coupled with annealing andthermal cycling which are believed to be lacking in conventionalmethods.

[0040] While the invention has been particularly shown and describedwith reference to the preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetails may be made without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A method with trench source to increase thecoupling of source to floating gate comprising the steps of: providing asubstrate having a source region; forming a split-gate flash memory cellon said substrate; forming a trench source in said source region;performing a source implant; forming a gate oxide layer over the insidewalls of said trench source; performing a lateral diffusion of saidsource implant; and performing thermal cycle of said substrate.
 2. Themethod of claim 1, wherein said substrate is silicon.
 3. The method ofclaim 1, wherein said trench source has tilted walls.
 4. The method ofclaim 3, wherein said tilted walls have an included angle between about10 to 45 degrees.
 5. The method of claim 1, wherein said source implantcomprises phosphorous (P) ions at a dosage level between about 1×10¹⁵ to1×10¹⁶ atoms/cm² and energy between about 10 to 50 KeV.
 6. The method ofclaim 1, wherein said source implant is performed at a tilt anglebetween about 10 to 45 degrees.
 7. The method of claim 1, wherein saidgate oxide layer has a thickness between about 4 to 70 Å.
 8. The methodof claim 1, wherein said lateral diffusion of said source implant isaccomplished at a temperature between about 850 to 950° C.
 9. The methodof claim 1, wherein said thermal cycle is performed between temperaturesabout 850 to 950° C.
 10. A method with trench source to increase thecoupling of source to floating gate comprising the steps of: providing asubstrate having active and passive regions defined; forming a firstgate oxide layer over said substrate; forming a first polysilicon layerover said gate oxide layer; forming a nitride layer over said firstpolysilicon layer; patterning said nitride layer to expose a portion ofsaid first polysilicon layer to define a floating gate area; performingoxidation of said portion of said first polysilicon layer to form apolyoxide layer over said first polysilicon layer; etching said firstpolysilicon layer using said polyoxide layer as a hard mask to form afloating gate; forming an interpoly oxide over said polyoxide; forming asecond polysilicon layer over said interpoly oxide; patterning saidsecond polysilicon layer to form a control gate; forming a trench sourcein said substrate; performing a source implant; forming a second gateoxide layer over the inside walls of said trench source; performing alateral diffusion of said source implant; and performing thermal cycleof said substrate.
 11. The method of claim 10, wherein said substrate issilicon.
 12. The method of claim 10, wherein said forming said firstgate oxide layer is accomplished by thermal growth at a temperaturebetween about 800 to 950° C.
 13. The method of claim 10, wherein saidfirst gate oxide layer has a thickness between about 70 to 90 angstroms(Å).
 14. The method of claim 10, wherein said forming said firstpolysilicon layer is accomplished with silicon source SiH₄ using LPCVDat a temperature between about 530 to 650° C.
 15. The method of claim10, wherein said first polysilicon layer has a thickness between about900 to 1100 Å.
 16. The method of claim 10, wherein said forming saidnitride layer over said first polysilicon layer is accomplished by CVDat a temperature between about 650 to 800° C. by reacting dichlorosilane(SiCl₂H₂) with ammonia (NH₃).
 17. The method of claim 10, wherein thethickness of said nitride layer is between about 700 to 900 Å.
 18. Themethod of claim 10, wherein said oxidation of said first polysiliconlayer to form poly-oxide is accomplished through thermal oxidation at atemperature between about 800 to 950° C.
 19. The method of claim 10,wherein said polyoxide layer has a thickness between about 1100 to 1300Å.
 20. The method of claim 10, wherein said interpoly oxide comprises alayer of first thermal oxide, a layer of high temperature oxide (HTO),and a layer of second thermal oxide.
 21. The method of claim 20, whereinsaid first thermal oxide layer has a thickness between about 30 to 50 Å,and is thermally grown at a temperature between about 800 to 950° C. 22.The method of claim 20, wherein said HTO layer has a thickness betweenabout 120 to 140 Å, and is deposited at a temperature between about 800to 950° C.
 23. The method of claim 20, wherein said second thermal oxidelayer has a thickness between about 60 to 80 Å, and is thermally grownat a temperature between about 800 to 950° C.
 24. The method of claim10, wherein said forming said second polysilicon layer is accomplishedwith silicon source SiH₄ using LPCVD at a temperature between about 530to 650° C.
 25. The method of claim 10, wherein said second polysiliconlayer has a thickness between about 1900 to 2100 Å.
 26. The method ofclaim 10, wherein said trench source has a depth between about 200 to600 Å.
 27. The method of claim 10, wherein said trench source has tiltedwalls.
 28. The method of claim 27, wherein said tilted walls have anincluded angle between about 10 to 45 degrees.
 29. The method of claim10, wherein said source implant comprises phosphorous (P) ions at adosage level between about 1×10¹⁵ to 1×10¹⁶ atoms/cm² and energy betweenabout 10 to 50 KeV.
 30. The method of claim 10, wherein said sourceimplant is performed at a tilt angle between about 10 to 45 degrees. 31.The method of claim 10, wherein said second gate oxide layer isthermally grown at a temperature between about 800 to 950° C.
 32. Themethod of claim 10, wherein said second gate oxide layer has thicknessbetween about 60 to 80 Å.
 33. The method of claim 10, wherein saidlateral diffusion of said source implant is accomplished at atemperature between about 800 to 950° C.
 34. The method of claim 10,wherein said thermal cycle is performed between temperatures about 800to 950° C.
 35. A split-gate flash memory cell having a trench sourcewith tilted walls comprising: a substrate having a source region; asplit-gate flash memory cell on said substrate; a trench source in saidsource region; a gate oxide layer over the inside walls of said trenchsource; and a laterally enlarged diffused area of said source region.36. The split-gate flash memory cell of claim 35, wherein said trenchhas a depth between about 200 to 600 Å.
 37. The split-gate flash memorycell of claim 35, wherein said trench has tilted walls.
 38. Thesplit-gate flash memory cell of claim 35, wherein said tilted walls havean included angle between about 10 to 45 degrees.
 39. The split-gateflash memory cell of claim 35, wherein said trench source is implantedat a tilt angle between about 10 to 45 degrees.
 40. The split-gate flashmemory cell of claim 35, wherein said gate oxide layer has a thicknessbetween about 60 to 80 Å.
 41. The split-gate flash memory cell of claim35, wherein said laterally enlarged diffused area spans at leastone-half the width of said floating gate.